Answers

Doing this fight now and he seems to have disappeared for good the second time he used that ability. I've circled the entire arena many times and he's nowhere to be found. No mist this time either. Bug?

EDIT: I might have killed him? The AoE snow things don't have any effect on me now, I think they're supposed to damage or debuff or something but I'm just standing in the room letting them fall on me and nothing happens.

EDIT2: Meh, I restarted it and it didn't bug out this time. Thanks for letting people restart from that point, ANet, it's refreshing that I didn't have to do the entire instance again.